1. Field of the Invention
The present invention relates to a process for production of a field-emission cold cathode having emitter electrodes each with an acute from end and particularly to an improved method for applying a protective sheet onto a wafer having a plurality of field-emitters formed at the surface, to later divide the cathodes into individual devices by dicing. The present invention also relates to a protective sheet suitably used in the method.
2. Description of the Related Art
"Field-emission cold cathode", was developed as an electron emitting source replacing "thermionic cathode" which emits electrons upon application of heat. Field-emission cold cathode emits electrons into air by quantum-mechanical tunneling which takes place when a strong electric field (2 to 5E7 V/cm or more) is generated at the acute front end of each emitter electrode. Therefore, the property of field-emission cold cathode is dependent upon the acuteness of emitter front end of each field-emitter, and it is said that the emitter front end is required to have a radius of curvature of several hundreds of angstrom (A) or less. Further, to generate an electric field such as above, it is necessary to arrange the two electrodes (emitter electrode and gate electrode) of field-emitter in close vicinity to each other (the distance between them is about 1 .mu.m or less) and applying a voltage of several tens to several hundreds volts (V). The surface cleanness of emitter electrode affects its work function as well and is an important factor for determining the emittability of the electrode.
In actual application of field-emission cold cathode, several thousands to several tens of thousands of field-emitters are formed on a wafer and used as an array where they are connected in parallel, in many cases. Therefore, utilizing the technique used for fine processing of semiconductor produces them.
As an actual production process of field-emission cold cathode, there is a process developed by Spindt et al. of SRI (Stanford Research Institute) of U.S.A., described in J. Appl. Phys. 39, p. 3504, 1968. In the process, a refractory metal (e.g. molybdenum) is deposited on a conductive substrate to form a structure having an acute front end. The process is shown in FIG. 26.
First, on a silicon substrate 71 is formed an oxide film as an insulating layer 72. Subsequently, molybdenum is vacuum-deposited as a gate layer 74. Then, a resist 73 is applied, and photolithography and etching are conducted to form an aperture 77 having a diameter of about 1 .mu.m. The insulating layer 72 is etched via the aperture 77 [FIG. 26(a)]. Thereafter, oblique deposition with rotation is conducted to form a sacrifice layer 78 made of aluminum. Then, molybdenum is vacuum-deposited in a vertical direction to form an emitter electrode 75 and a molybdenum film 76 [FIG. 26(b)]. Lastly, the sacrifice layer 78 is subjected to selective etching to lift off the molybdenum film 76 formed on the sacrifice layer 78, whereby a device structure is obtained [FIG. 26(c)].
In the device produced by the above process, when a voltage is applied so that the emitter electrode 75 becomes negative and the gate electrode 74 becomes positive, electrons are emitted from the front end of the emitter electrode 75 into a direction perpendicular to the substrate 71. Such a structure is generally called a vertical type field-emitter.
The applications of such a field-emission cold cathode include use as electron sources such as electron tube or the like. In the above process, as in the process generally used for production of semiconductor device, several hundreds to several thousands of devices are produced simultaneously in one wafer. In order to mount these devices on an electron tube, the devices must be divided into individual devices by dicing.
Dicing is a step of cutting a wafer by the use of a grindstone (an abrasive such as diamond, C-BN or the like) rotating at a high speed. Dicing is generally conducted by injecting cutting water on the cutting area of wafer for the purpose of cooling and prevention of sludge scattering. However, the cutting water flows on the wafer surface having devices formed and carries sludge (of silicon wafer and conductive substance such as electrode material or the like) to the vicinity of each emitter electrode. Remaining of sludge in the vicinity of emitter invites deterioration of the insulating property of emitter and impairs the reliability of device.
For alleviation of the above problems, it has been conducted to cover, at the time of dicing, a wafer with a protective sheet comprising a resin base material and an adhesive coated thereon, whereby the insulating property of emitter has been maintained at a satisfactory level.
The general procedure of the dicing step is described below.
First, onto the wafer surface after emitter formation and lift-off is applied a protective sheet comprising a base material and an UV-curing adhesive coated thereon. An adhesive sheet is applied onto the backside of the wafer, and dicing is conducted at the protective sheet side. In dicing, the cutting water containing sludge is injected onto the wafer surface; however, the presence of the protective sheet prevents direct contact of the water with the emitter area, whereby no defects such as poor insulation caused by dust, stain and the like arise.
After dicing, the protective sheet is peeled from the wafer. This peeling can be conducted easily, for example, by using an UV-curing adhesive as the adhesive of the protective sheet and applying an ultraviolet light onto the protective sheet surface to cure the adhesive and lower its adhesivity.
However, the adhesive of the protective sheet flows on the wafer by the pressure applied to the protective sheet for its application onto the wafer. As a result, as shown in FIG. 24, at the area where an emitter is formed, the adhesive of a protective sheet 22 comprising a base material 23 and an adhesive layer 24 penetrates into the aperture 77 of a gate electrode 74 and contacts with the front end of an emitter electrode 75. The depth of penetration of adhesive is about 0.25 .mu.m when the gate aperture has a diameter of about 0.6 .mu.m (an example of the measurement of penetration depth H (.mu.m) by the present inventors was H=D.times.0.3+0.07 when the gate diameter D was 0.6 to 1.6 .mu.m). By peeling the protective sheet after dicing, the adhesive is removed and remaining of adhesive is not detectable even by SEM observation or the like. However, a very small amount of the adhesive remains on the emitter surface actually, which has increased the effective work function of emitter surface and has deteriorated the property of device.
FIG. 25 shows the emission properties of (1) a device produced by the above standard process (Adhesive Free: defined as ".largecircle.") and (2) a device also produced by the above standard process (in this case, application of protective sheet, and peeling of the sheet were conducted to examine the effect of remaining adhesive, Adhesive Remained: defined as ".circle-solid."). The emission property of the latter device formed using a protective sheet was apparently inferior to that of the former device.
Japanese Patent Application Kokai (Laid-Open) No. 356942/1992 discloses the followings. When an adhesive layer is formed selectively on the areas of protective sheet slightly larger than the areas to be cut, the resulting protective sheet is applied onto a wafer, and dicing is made at the protective sheet side using a dicing machine, or when an UV-curing type adhesive layer is formed on the whole surface of protective sheet, a light-shielding mask is placed only on the areas of protective sheet slightly larger than the areas to be cut, an ultraviolet light is applied onto the protective sheet to cure the adhesive layer at the areas other than the areas to be cut, the resulting protective sheet is applied onto a wafer, and dicing is made at the protective sheet side using a dicing machine, there is no staining of the device surface by the adhesive and, because the protective sheet is bonded to the wafer only at the areas to be cut and the vicinities, there is no staining of the device surface by sludge, etc.
When the above practice is applied in production of field-emission cold cathode, it is presumed that no adhesive contacts with the front end of emitter electrode and that the reduction in emitter property caused by remaining of adhesive is prevented. However, formation of adhesive layer only on the areas of protective sheet slightly larger than the areas to be cut may cause partial peeling of applied protective sheet owing to the pressure applied thereto during dicing, the cutting water may penetrate into the device from the peeled area during dicing, and the intended purpose may not be achieved.
In order to prevent the above partial peeling of applied protective sheet, it is considered to use an adhesive of high adhesivity in formation of a protective film. The protective sheet used for dicing generally comprises a synthetic resin film having a thickness of about 50 to 100 .mu.m and an adhesive layer formed thereon, and its rigidity is not so high. Usually, such a protective sheet is delivered from the supplier in the form of a roll and is used by drawing from the roll. When the protective sheet is drawn from its roll or subjected to looseness removal or aligning, the thin protective sheet, which shows considerable expansion and contraction, tends to expand towards a direction in which the protective sheet receives a force and contracts in a direction perpendicular thereto. When the adhesive layer is formed only on the areas of protective sheet slightly larger the areas to be cut, for example, in a width of the cutting width plus 60 .mu.m (30 .mu.m at one side of the cutting width) as described in Example of the above-mentioned literature, this 60 .mu.m corresponds to 0.06% of wafer diameter when a wafer of 4 in. (10 cm) in diameter is used; when such a protective sheet is applied onto such a wafer, aligning at an accuracy of 0.06% is possible for one particular point of the wafer but is absolutely impossible for the whole wafer surface in view of the expansion and contraction of the protective sheet. Further, the wafer size is increasingly becoming larger, which makes the accuracy of aligning even more difficult. Misalignment and consequent adhesive absence on the areas to be cut or consequent presence of cured adhesive on the areas to be cut invites the penetration of cutting water into device and resultant reduction in emitter property.
Therefore, the width of adhesive application onto protective sheet must be large. However, when the width is large, the adhesive layer may cover the emitter area owing to the misalignment caused by the expansion and contraction of protective sheet. Thus, alignment conducted in application of protective sheet onto wafer is very difficult.